mike stehr Posted December 18, 2015 Share Posted December 18, 2015 (edited) I found this schematic when I was pondering over a single-ended 50C5 amplifier circuit with a power supply xformer for isolation. I found a little RCA SE 50C5 phono console amplifier, and the unit was powered straight off the wall. I'm pretty sure this circuit was drawn up by Joe Rosen. Dunno if the guy is around anymore, or if he actually built a finished amplifier from this schematic. He used to do some brutally honest takes on various brands of vintage tube amplifiers on Audio Asylum in the day...humorous and colorful. Any thoughts on the circuit? Seems pretty basic to me... Not really planning on jumping into the project, just mere pondering. Mike Edited December 18, 2015 by mike stehr Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted December 18, 2015 Share Posted December 18, 2015 That's an interesting arrangement in quite a number of respects. Has there been any discussion on what this amp sounds like? Maynard Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 18, 2015 Author Share Posted December 18, 2015 (edited) I don't know if anyone has built the amplifier circuit, or if there was any discussion if anyone had. I remember coming across the schematic the first time around ten years ago. At the time the schematic was on his webpage, or blog or some such. He had a write-up, explaining why and what he did regarding the circuit. I if recall, (not sure) Joe wasn't a fan of directly heated single-ended triodes. I think this was his contribution to a single-ended amplifier. I don't even know if he meant the circuit as a joke or not. With a name of Cancerous-Tumortronix "Fleabreeze" Kowabunga 2300BS DC, it makes one wonder... Maybe I can dig up some more information on the circuit. Edited December 18, 2015 by mike stehr Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 19, 2015 Author Share Posted December 19, 2015 (edited) Here's all I can really find concerning the Fleabreeze amp. The OP has the link to Joe's webpage in his first post, but now it is dead. http://www.diyaudio.com/forums/tubes-valves/161193-flea-thingy-sounds-good.html I'm registered on diyaudio, but can't remember the password, so I can't see the schematic or any photos of the guy's project who started the thread. Not much to really glom from the eight pages of thread. One guy wasn't too thrilled about grid bias for some reason. Typical diyaudio banter... Edited December 19, 2015 by mike stehr Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 6, 2017 Author Share Posted January 6, 2017 Pondering over the Fleabreeze again lately, I registered on DIYaudio. As far as discussion of sound, the only guy that seems to have built a completed amplifier is the only one that can describe the sound. He likes the sound of course. The builder is from Argentina so he had rolled input tubes of various types until he could source a HP 5751. Not that easy to just buy tubes cheap in Argentina. In spite of other posters trying to convince him to get away from grid bias for the usual cathode resistor approach, and resisting the idea of using a full-wave bridge, I have to admire the guy for sticking with the Rosen schematic right down to the Hewlett-Packard 5751 tube. I'm sure Joe just used what was on hand at the time... The P-T31 OPT is a 13-14 dollar part from AES, or I could use the bacteria-sized OPTs from the little RCA 50C5 SE console amp. I could use the RCA amplifier as a test chassis, and I have a big Magnetek isolation transformer good for over 2 amps to use for safety. Then maybe if things sound good, scrounge up a chassis and buy the Triad N68X isolation transformer for use with a complete built amp. I have a dual-ganged 250K stereo pot to use, and probably could scrounge up a few proper values for resistors. I'm pretty sure I have the proper value coupling caps on hand. I'd have to order 680uF electrolytics, and probably better off going with 250 volt for a bit more safety. Might not be a bad amplifier for a PC/media set-up... Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 7, 2017 Share Posted January 7, 2017 If you look at the schematic you will see that the opt (which is 5k:8) is connected to a 4 ohms speaker to give the 50C5 its intended load impedance of 2.5k. So, that opt is not going to be a great choice even though it would work. I don't see the advantage of biasing the tube a little higher than its usual -7.5V unless he did it to cool the tube off a bit. I called AES about the xfmr last year and they have no specs on its frequency response, or the maximum bias that it can handle without saturating. Given its size, and assuming it's air gapped, I'd hope it can handle 50ma as they say it can be used with a 6V6. But, since you have the tubes, why not build the version of the Little Gem that Marty did? And having an isolation xfmr makes it very easy. Put a pair of inexpensive Edcors on it and you will have a nice 1 wpc amp which will surprise you with the CWs. Maynard Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 15, 2017 Author Share Posted January 15, 2017 At the moment there is no extra funds for output transformers like Edcors. But it won't keep myself from pondering something to the regard like a little Gem amplifier. The isolation transformer I have is big 10 pound beast, more of a bench item. I need to keep digging through the weird tubes and find some more AA5 tubes... For now I returned to Magnavox 6V6 PP mono amplifiers, to see if I can get them to sound a little better than they do. A shield for the 12AX7 should help with noise, being so close to the OPTs...amongst other things. The amps look too nice to just sound meh... Quote Link to comment Share on other sites More sharing options...
thebes Posted January 16, 2017 Share Posted January 16, 2017 Hey Mike, I've got a pair of Edcor XSE15-8-K's from a failed project you could have. I've also pretty sure I've got some 5751's and maybe a 50C5. That isolation xformer is a $20 item off Amazon. Quote Link to comment Share on other sites More sharing options...
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